uwp - 如何在 UWP 中数据绑定(bind)到 RichEditBox 的纯文本值？

Question

使用 UWP 中的普通 TextBox，您可以数据绑定(bind)到 Text 属性，并轻松地从 ViewModel 获取或设置值。 RichEditBox虽然没有可绑定(bind)数据的 Text 属性；相反，您必须使用 ITextDocument interface由 Document 属性公开，并使用各种方法来获取和设置文本。

如何将纯文本数据绑定(bind)到我的 ViewModel 中的内容？

Answer 1

可以使用自定义 attached property 对 RichEditBox 的纯文本进行数据绑定(bind).此附加属性处理文档的富文本和纯文本之间的转换。

这是一个示例 XAML 页面、代码隐藏和显示附加属性用法的 ViewModel:

XAML

将此复制为项目中新页面的内容

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
  <StackPanel Margin="30">
    <RichEditBox local:RichEditBoxExtension.PlainText="{Binding PlainText,
      Mode=TwoWay}" x:Name="richedit"/>
    <Button Content="Bold selection" Click="MakeBold"/>
    <Button Content="Change plain text (view model)" Click="ChangeText"/>
    <Button Content="Change rich text (control property)" Click="ChangeRichText"/>
    <TextBlock Text="PlainText property is..." />
    <TextBlock Text="{Binding PlainText, Mode=OneWay}" />
  </StackPanel>
</Grid>

代码隐藏

这假设您使用的是默认的 MainPage.xaml.cs；适当更改构造函数名称

public MainPage()
{
  InitializeComponent();
  DataContext = model = new ViewModel();
  model.PlainText = "Hello, world";
}

private void ChangeText(object sender, RoutedEventArgs e)
{
  model.PlainText = "Here is some plain text";
}

private void ChangeRichText(object sender, RoutedEventArgs e)
{
  richedit.Document.SetText(TextSetOptions.None, "Here is some rich text");
  var selection = richedit.Document.Selection;
  selection.StartPosition = 8;
  selection.EndPosition = 12;
  selection.CharacterFormat.Underline = UnderlineType.Single;
  selection.MoveStart(TextRangeUnit.Word, 1);
  selection.Expand(TextRangeUnit.Word);
  selection.CharacterFormat.Weight = FontWeights.Bold.Weight;
}

private void MakeBold(object sender, RoutedEventArgs e)
{
  richedit.Document.Selection.CharacterFormat.Weight = FontWeights.Bold.Weight;
}

View 模型

没什么特别的；只是一个字符串属性。您可以将它放在自己的文件中，或将其粘贴到主代码隐藏文件中。

public class ViewModel : INotifyPropertyChanged
{
  public event PropertyChangedEventHandler PropertyChanged;
  string plainText;
  public string PlainText
  {
    get { return plainText; }
    set
    {
      plainText = value;
      RaisePropertyChanged();
    }
  }

  void RaisePropertyChanged([CallerMemberName] string propertyName = "")
  {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }
}

到目前为止，没有什么特别的。 RichEditBox 使用附加属性 RichEditBoxExtension.PlainText 并将其绑定(bind)到 ViewModel 属性 PlainText。页面上还有另一个 TextBlock 用于显示 PlainText 属性的当前值，以及一些用于操作文本的按钮。

RichEditBoxExtension.PlainText 的实现非常简单，但由于依赖属性基础结构和需要避免无休止的属性更新(改变 rich文本触发纯文本，富文本触发富文本，富文本触发纯文本，依此类推。

附加属性

这可以在它自己的文件中，也可以再次粘贴到代码隐藏文件中。

public class RichEditBoxExtension
{
  // Standard attached property. It mimics the "Text" property of normal text boxes
  public static readonly DependencyProperty PlainTextProperty =
    DependencyProperty.RegisterAttached("PlainText", typeof(string),
    typeof(RichEditBoxExtension), new PropertyMetadata(null, OnPlainTextChanged));

  // Standard DP infrastructure
  public static string GetPlainText(DependencyObject o)
  {
    return o.GetValue(PlainTextProperty) as string;
  }

  // Standard DP infrastructure
  public static void SetPlainText(DependencyObject o, string s)
  {
    o.SetValue(PlainTextProperty, s);
  }

  private static void OnPlainTextChanged(DependencyObject o, 
    DependencyPropertyChangedEventArgs e)
  {
    var source = o as RichEditBox;
    if (o == null || e.NewValue == null)
      return;

    // This attaches an event handler for the TextChange event in the RichEditBox,
    // ensuring that we're made aware of any changes
    AttachRichEditBoxChangingHelper(o);

    // To avoid endless property updates, we make sure we only change the RichText's 
    // Document if the PlainText was modified (vs. if PlainText is responding to 
    // Document being modified)
    var state = GetState(o);
    switch (state)
    {
      case RichEditChangeState.Idle:
        var text = e.NewValue as string;
        SetState(o, RichEditChangeState.PlainTextChanged);
        source.Document.SetText(Windows.UI.Text.TextSetOptions.None, text);
        break;

      case RichEditChangeState.RichTextChanged:
        SetState(o, RichEditChangeState.Idle);
        break;

      default:
        Debug.Assert(false, "Unknown state");
        SetState(o, RichEditChangeState.Idle);
        break;
    }
  }

  #region Glue

  // Trivial state machine to determine who last changed the text properties
  enum RichEditChangeState
  {
    Idle,
    RichTextChanged,
    PlainTextChanged,
    Unknown
  }

  // Helper class that just stores a state inside a textbox, determining
  // whether it is already being changed by code or not
  class RichEditChangeStateHelper
  {
    public RichEditChangeState State { get; set; }
  }

  // Private attached property (never seen in XAML or anywhere else) to attach
  // the state variable for us. Because this isn't used in XAML, we don't need
  // the normal GetXXX and SetXXX static methods.
  static readonly DependencyProperty RichEditChangeStateHelperProperty =
    DependencyProperty.RegisterAttached("RichEditChangeStateHelper",
    typeof(RichEditChangeStateHelper), typeof(RichEditBoxExtension), null);

  // Inject our state into the textbox, and also attach an event-handler
  // for the TextChanged event.
  static void AttachRichEditBoxChangingHelper(DependencyObject o)
  {
    if (o.GetValue(RichEditChangeStateHelperProperty) != null)
      return;

    var richEdit = o as RichEditBox;
    var helper = new RichEditChangeStateHelper();
    o.SetValue(RichEditChangeStateHelperProperty, helper);

    richEdit.TextChanged += (sender, args) =>
    {
      // To avoid re-entrancy, make sure we're not already changing
      var state = GetState(o);
      switch (state)
      {
        case RichEditChangeState.Idle:
          string text = null;
          richEdit.Document.GetText(Windows.UI.Text.TextGetOptions.None, out text);
          if (text != GetPlainText(o))
          {
            SetState(o, RichEditChangeState.RichTextChanged);
            o.SetValue(PlainTextProperty, text);
          }
          break;

        case RichEditChangeState.PlainTextChanged:
          SetState(o, RichEditChangeState.Idle);
          break;

        default:
          Debug.Assert(false, "Unknown state");
          SetState(o, RichEditChangeState.Idle);
          break;
      }
    };
  }

  // Helper to set the state managed by the textbox
  static void SetState(DependencyObject o, RichEditChangeState state)
  {
    (o.GetValue(RichEditChangeStateHelperProperty) 
      as RichEditChangeStateHelper).State = state;
  }

  // Helper to get the state managed by the textbox
  static RichEditChangeState GetState(DependencyObject o)
  {
    return (o.GetValue(RichEditChangeStateHelperProperty) 
      as RichEditChangeStateHelper).State;
  }
  #endregion
}

附加属性基本上做了两件事，但是围绕着它有很多样板代码和状态机:

1. 当 PlainText 附加属性更改时，它会使用 source.Document.SetText(TextSetOptions.None, text)< 将 RichEditBox 更新为纯文本
2. 当 RichEditBox 文本更改(包括富文本更改)时，它会使用 richEdit.Document.GetText(TextGetOptions.None, out text) 然后是 o.SetValue(PlainTextProperty, text)。

请注意，这种基本方法可用于数据绑定(bind)您要根据实际数据可绑定(bind)属性计算的其他“派生”属性。